Silver removal apparatus and method

ABSTRACT

Silver is recovered from solutions such as photographic fixer solutions by contacting the solution with a metallic exchange medium. The metallic exchange medium comprises a plurality of buoyant elements, each comprising a substrate coated with iron. The substrate can be expanded polystyrene beads and the iron can be sponge iron of small particle size.

RELATED APPLICATION

This application is related to my copending application entitled "MetalRemoval Apparatus and Method," filed on the same day as thisapplication, Ser. No. 266,048, which is incorporated herein by thisreference.

BACKGROUND

This invention is directed to removing and preferably recovering silverfrom solutions such as used photographic fixer solutions.

Dissolved silver is commonly removed from solutions to recover thesilver as a valuable product and/or to remove the silver as a pollutant.For example, silver is removed from photographic solutions to recoverthe silver as a valuable product as well as to permit the solution to bedischarged without causing pollution.

Numerous devices have been developed for removing silver from usedphotographic fixer solutions. Such devices are described in U.S. Pat.Nos. 3,369,801; 3,692,291; 4,035,181; and 4,156,604; all of which areincorporated herein by this reference.

These devices utilize an electron exchange reaction, i.e., anelectrochemical exchange reaction which occurs between a more noblemetal and a less noble metal in order to recover the silver in the usedsolution. The less noble metal in these devices is iron in the form ofinexpensive steel wool or a window screen. When the silver rich solutionis passed over the iron, an electron exchange occurs wherein the ironmetal Fe⁰ is oxidized to ferric or ferrous ion and the silver ion (Ag⁺)is reduced to silver metal (Ag⁰). The silver precipitates from thesolution to form a sludge, from which the silver is recovered. The spentsolution from the device is discharged to a sewer or returned to afixing tank.

The use of steel wool or window screen as the silver source hassignificant disadvantages. Among these problems is inefficient use ofthe iron, a substantial portion of the iron not being used to replacesilver. Further, frequent clogging and channeling have been experienced.Channeling is a particular problem as the iron in the steel wool orwindow screen is depleted, resulting in voids forming in the exchangemedium. These voids provide a path of lower resistance to flow and lossof efficiency in silver recovery and material use.

A further disadvantage is that these units can require the use of steelwool or window screen of different sizes or weights dependent upon thesilver content or pH of the solution being treated. This can require theavailability of two or more types of silver recovery devices, or the useof a plurality of devices.

Another disadvantage of existing recovery devices is that it isdifficult to purify the recovered silver. The recovered silver is mixedin with large particles of window screen or steel wool. In order torefine the silver, it is first necessary to comminute the silver sludgecontaining particulate iron into a fine powder, such as by ball milling.This is a time-consuming and costly step.

A further disadvantage of the existing devices is that leakage betweenthe container lid and body can occur. A monocoque container cannot beused because the steel wool or window screen needs to be placed in thecontainer. Such leakage can cause significant property damage.

In view of these problems, it is apparent that there is a need for asilver recovery apparatus that makes efficient use of the exchangemedium, avoids clogging and channeling in use, does not leak, maintainsgood contact with the silver-containing solution, can be used for a widevariety of solutions having different silver contents and different pH,and recovers silver in a form that does not require comminution forrefining.

SUMMARY

The present invention is directed to an apparatus having these features.The apparatus is useful for recovering silver from a solution, such as aphotographic fixer solution. The apparatus comprises a container formedof a material substantially non-reactive with the solution. Thecontainer has an inlet for introducing influent solution into thecontainer and an outlet for removing spent solution from the container,the spent solution having silver removed therefrom. The containercontains a metallic exchange medium that comprises a plurality ofbuoyant elements. The elements are sufficiently buoyant to float in thesilver-containing solution. Each element comprises a substrate coatedwith iron, the substrate being substantially non-reactive with thesolution.

A preferred substrate is a polymeric substrate such as expandedpolystyrene beads, which are hollow and contain air. Preferably the ironis sponge iron, secured to the substrate with an adhesive. Preferablythe sponge iron is from about +150 mesh to about -10 mesh, and morepreferably from about +80 mesh to about -20 mesh. Preferably the beadsare from about 1/8 to about 1/2 inch, and more preferably from about 1/4to about 3/8 inch in diameter. Preferably the iron used is substantiallyfree of zinc, aluminum, and copper to insure that these pollutants donot go into solution.

Preferably, the container is not filled with the exchange medium, sothat a void can be left at the bottom of the container for precipitatedsilver to collect. This can be achieved by using an amount of metallicexchange medium so that the medium comprises from about 70% to about 90%of the volume of the container.

The precipitated silver can easily be recovered by withdrawing thecontents of the container and pyrolyzing the polymeric substrate in asingle heating step. This leaves substantially only iron and silverparticles that can be refined without comminution.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is a perspective view, partially broken away, of an apparatus forrecovering silver according to the present invention;

FIG. 2 is a longitudinal sectional view of the apparatus of FIG. 1 takenon line 2--2 of FIG. 1; and

FIG. 3 is a sectional view of one of the buoyant elements of themetallic exchange medium used with the apparatus of FIG. 1.

DESCRIPTION

The present invention is directed to an apparatus and a method forremoving silver from solutions. Although the present invention will beprincipally described with regard to removing silver from usedphotographic fixing solutions, the apparatus and method can also be usedfor removing silver from solutions other than photographic fixingsolutions.

With reference to FIGS. 1 and 2, an apparatus 10 for removing silverfrom photographic fixing solutions comprises a pail shaped container 12that includes a monocoque body portion 14 forming the side and bottomwalls of the container 12, and a top lid 16. The top edge of the bodyportion has an integral strengthening rib 18 over which the lid 16 snapfits. There is a gasket 20 between the rib 18 and the lid 16 to preventleakage of photographic fixing solution from the container 12.

There are two holes or ports through the lid 16, an inlet hole 22 in thecenter of the lid and an outlet hole 24 toward the outer edge of thelid. The inlet hole and the outlet hole are provided with a gasketedplastic fitting 26 and 27, respectively, with a gasket 29 on theunderside of the lid to prevent leakage of photographic fixing solutionaround the outside of the fittings 26 and 27. Both gasketed fittings 26and 27 are connected outside of the container 12 by means of plastictubing 28 to a rotatable T-connector 30. The T-connector 30 for theinlet port 22 is connected to a length of tubing 32 through whichinfluent liquid passes. Likewise, the T-connector 30 for the outlet port24 is provided with tubing 33 for passage of the effluent from thecontainer 12. The T-connectors are rotatable so that influent andeffluent liquid can pass to and from the container in any desireddirection.

The two T-connectors 30 are joined by an inverted V-shaped assembly 36comprising two pieces of tubing 38, each piece of tubing being connectedat one end to one of the T connectors 30 and at the other end to a 90°elbow fitting 40, which is at a higher elevation than the elevation ofthe inlet 22 and the outlet 24. An anti-siphon hole 41 is provided atthe apex of the elbow fitting 40. The assembly 36 serves as an emergencygravity bypass for rapid discharge of influent solution during flowsurges or in case there is a plug in the container 12.

The inlet plastic fitting 26 has connected thereto on the inside of thecontainer 12 a vertically oriented tube 42 which extends almost to thebottom of the container 12. In the container is a metallic exchangemedium 44. Both the bottom of the inlet tube 42 and the outlet fitting27 are provided with a fine mesh screen or strainer 46 made ofsufficiently small mesh to prevent the metallic exchange medium 44 fromplugging the inlet tube 42 or passing out of the container through theoutlet 24.

The container can be any size convenient to handle, and generally fromabout 1 to 10 gallons, and more preferably from about 3 to about 7gallons. Alternatively, rather than using a container 12 that issuitable for only one use, a reusable column that can be repeatedly berefilled with metallic exchange medium 44 can be used. For example, a51/2 feet tall column 13 inches in diameter is satisfactory. Such a talland narrow column is advantageous in that influent liquid contacts alarger amount of exchange medium 44 than if the container were short andwide.

Rather than using a container 12 comprising a body portion 14 and a lid16, the container can be provided as a single integral monocoquestructure. This has the advantage that leaks cannot occur between thebody portion 14 and the lid 16 as can occur with prior art units. Byusing the metallic exchange medium 44 rather than steel wool or windowscreen, it is possible to fill a sealed container through the inlet 22and/or outlet 24.

The choice of the materials for the apparatus 10 is important, in thatall of the components of the apparatus, with the exception of themetallic exchange medium 44, need to be substantially non-reactive withthe influent liquid. For photographic fixing solutions, preferably allmaterials used are polymeric. For example, the container 12, includingthe body portion 14 and the lid 16, is formed of a plastic material suchas polyethylene, polypropylene, polystyrene, nylon, orpolyvinylchloride. This can be done by forming the container 12 fromsuch a material, or forming the container from a reactive material whichis a lined with a non-reactive plastic material. Likewise, the fittings26 and 27, tubing 32, 34, 38, and 42, as well as the T-connectors 30 andelbow 40 are formed of a substantially non-reactive material such aspolyvinylchloride. The gasket 20 between the lid 16 and body portion 14of the container and the gasket of the gasketed plastic fittings 26 canbe formed of neoprene. The screens 46 can be formed of nylon orfiberglass, among other materials.

From FIGS. 1 and 2, it can be seen that liquid is introduced into thecontainer 12 at the bottom of the container in a void space 48. Theliquid then passes up through the metallic exchange medium 44 toward thetop of the container, to pass out from the container through the outlet24. An advantage of this configuration is that the influent liquid isdistributed evenly across the cross-sectional area of the container 12at the bottom, thereby insuring even flow upwardly through the metallicexchange medium 44 with minimum channeling occurring.

The exchange medium 44 comprises a plurality of bouyant elements 50, oneof which is shown in FIG. 3. The bouyant elements 50 can be of anyshape, including oval and spherical. Each element comprises a substrate52 coated with metal 54. The substrate 52 is chosen so that the elements50 are buoyant in the container when the container is filled with thesolution from which metal is being removed. As used herein, the term"buoyant" means that an element 50 floats in a solution that the elementis being used for treating.

The substrate is substantially non-reactive with the liquid beingtreated. For a photographic fixing solution, a non-reactive polymericmaterial such as polyethylene, nylon, polypropylene, polystyrene, orpolyvinylchloride can be used. Preferably the beads are hollow, andfilled with air 56 to insure buoyancy. The preferred substrate isexpanded polystyrene beads. Such beads are available from Vertex ofVernon, California, under Catalog Designation EPS.

The substrate preferably has a diameter of from about 1/8 inch to about1/2 inch, and, more preferably, of from about 1/4 inch to about 3/8inch. If the bouyant elements are too small, plugging of the metallicexchange medium can occur. If the bouyant elements are significantlylarger than about 3/8 inch to about 1/2 inch, then the rate of recoveryof silver is decreased due to less available surface area of themetallic exchange medium.

The metal 54 can be held on the beads 50 by means of an adhesive 55 suchas a synthetic organic elastomer type adhesive. A suitable syntheticorganic elastomer adhesive is available from Industrial Polychem, Inc.of Gardena, California, under Catalog No. 5031. The adhesive chosenneeds to be one that when cured does not attack the substrate, and isalso stable and substantially non-reactive when exposed to the liquidbeing treated. Since photographic fixer solutions can be acid oralkaline, the adhesive should be stable in both mild acid and mildalakline solutions.

For recovery of silver, the metal 54 can be a metal that is less noblethan silver such as aluminum, zinc, copper or iron. A disadvantage ofzinc, aluminum and copper is that each is a water pollutant in its ownright. Therefore, for recovery of silver, preferably iron is used.Preferably, the iron used is substantially free of zinc, aluminum, andcopper to avoid introducing pollutants into the solution being treated.Moreover, preferably the iron is substantially free of carbon, i.e.,steel is not used. This is because the presence of carbon can interferewith the recovery of silver. The recovery of silver depends upon theacidic photographic solution etching the metallic exchange medium,resulting in sloughing off of precipitated silver. If the iron containscarbon, less etching can occur, and the silver does not slough off,thereby preventing iron underlying deposited silver from entering intothe electrochemical exchange reaction.

Preferably, sponge iron is used. Preferably, the sponge iron is fromabout +150 mesh (does not pass through a 150 mesh sieve) to about -10mesh (passes through a 10 mesh sieve). This is because with particlessubstantially smaller than about 150 mesh, reduction in recovery ofsilver is noted. It is believed that this results from generation ofhydrogen due to the acidic photographic fixer solution quickly reactingwith the iron. Such quick reaction occurs because of the high surfacearea of the small iron particles. The generation of hydrogen can lowerthe pH of the photographic fixer solution, wich results in increasedusage of iron and undesirable break-up of the ammonium thiosulfatepresent in the photographic fixing solution.

When the iron particles are larger than about 10 mesh, there isinadequate surface area for quick and efficient recovery of silver.

In view of these considerations, more preferably the iron is from about+80 mesh to about -20 mesh.

The bouyant elements 50 can be formed by putting 1 gallon volume ofexpanded polystyrene particles in a cement mixer with 8 fluid ounces ofadhesive solution. The adhesive solution is formed by diluting 0.4 partsby volume adhesive 5031 from Industrial Polychem, Inc. from Gardena,California, with 0.06 parts by volume methyl ethyl ketone and 0.54 partsby volume hexane. The adhesive coating 53 and expanded polystyreneparticles 52 are mixed for about 11/2to 2 minutes. Then, the sponge iron54 is added, in an amount of 2 pounds per 1 gallon volume expandedpolystyrene beads. The cement mixer continues to run until the expandedpolystyrene particles are substantially completely coated with the iron,and then the adhesive is permitted to cure.

Preferably the container 12 is not completely filled with the metallicexchange medium 44 so that there is a void space 48 at the bottom foreven distribution of incoming liquid, and to provide a space forrecovered silver to collect. An adequate void space can be provided byusing an amount of metallic exchange medium having a volume from about70% to about 90% volume of the container 12.

To use the apparatus 10, influent liquid is introduced through tubing32, into the inlet tube 42 for discharge adjacent the bottom of thecontainer 12. The liquid percolates upwardly through the metallicexchange medium 44, where silver in the influent liquid undergoes anelectrochemical exchange reaction with the more noble second metalconstituting the reaction exchange medium 44. Spent liquid of a lowercontent of silver than the influent liquid is discharged through theoutlet 24.

The apparatus 10 is suitable for photographic fixer solutions. As usedherein, the term "photographic fixer solution " refers to both fixersolutions and bleach-fixer solutions. A fixer solution conventionally isa solution comprising water, sodium or ammonium thiosulfate, sodium orpotassium sulfite, sodium or potassium hydroxide, and acetic or sulfuricacid. It can also contain aluminum sulfate. A bleach-fixer solutioncontains the same constituents, but also includes ferric ammoniumethylene diamine tetracetic acid.

The optimum pH for recovering silver from a photographic fixing solutionis from about 4.5 to 5.5. The pH of an available photographic fixingsolution can be as low as 4 or, in the case of a bleach-fixer solution,as high as 8. It is possible to modify the pH of the influent liquid toa pH of from about 4.5 to about 5.5 for effective recovery of silver.

More than one container can be used. If desired, containers 10 can beconnected in series and/or in parallel.

In use, an apparatus 10 comprising a five-gallon container 12 canrecover about 120 ounces of silver from a photographic fixer solution atan influent rate of from about 300 to about 500 milliliters per minute,with about 95% to about 99% recovery of the incoming silver. This is asignificant improvement over comparable prior art devices using steelwool or window screen which can treat only a maximum of about 300milliliters per minute.

As the electrochemical exchange reaction occurs, the iron coating 54 isdepleted and silver precipitates on the exchange medium 44. As thereaction further precedes, the silver sloughs off into the void 48 atthe bottom of the container, forming a silver sludge.

The silver can be recovered by removing the contents of the containerfrom the container, then drying the contents and pyrolyzing thesubstrate. This can be effected by drying at an elevated temperaturesufficiently high to pyrolyze the plastic substrate 52 and the adhesivecoating 55. Materials such as polystyrene can be pyrolyzed in about 30minutes at a temperature of about 500° F. After drying and pyrolysis,which can be effected in a single heating step, all that remains is afine powder principally constituting particulate silver and small ironparticles that originally constituted the metallic coating 54 on thesubstrate 52. This powder can be refined to recover the silver accordingto conventional techniques, without the need for any furthercomminution.

The apparatus 10 provides substantial advantages compared to prior artdevices for recovering silver from photographic fixing solutions.Principally, more photographic fixing solution can be treated with theapparatus 10 than with comparable prior art devices. It is believed thatthis results from reduction in channeling, avoidance of plugging, andbetter contact between the fixing solution and the iron than is possiblewith prior art devices. A further advantage is that a sealed containercan be used, thereby minimizing chances that the container will leak.Further, the recovered product can be refined without the need for anexpensive and difficult comminution step.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. For example, rather than supplying an integral containercontaining the reaction medium 44, the reaction medium 44 can besupplied separately for use in a reusable container, such as the talland narrow column previously described. Therefore, the spirit and scopeof the appended claims should not necessarily be limited to thedescription of the preferred versions contained herein.

What is claimed is:
 1. Apparatus for removing silver from asilver-containing solution comprising a container formed of a materialsubstantially non-reactive with the solution, an inlet for introducingthe solution into the container, and an outlet for removing solutionwith silver removed therefrom from the container, the containercontaining a metallic exchange medium comprising a plurality of buoyantelements each comprising a substrate coated with iron, the substratebeing substantially non-reactive with the solution.
 2. The apparatus ofclaim 1 in which the substrate is polymeric.
 3. The apparatus of claim 2in which the substrate comprises expanded polystyrene beads.
 4. Theapparatus of claim 1 or 3 in which the iron is sponge iron.
 5. Theapparatus of claim 4 in which the sponge iron is of a particle size offrom about +150 mesh to about -10 mesh.
 6. The apparatus of claim 5 inwhich the sponge iron is of a particle size of from about +80 mesh toabout -20 mesh.
 7. The apparatus of claim 1 in which the volume of themetallic exchange medium comprises from about 70 to about 90% of thevolume of the container.
 8. The apparatus of claim 1 in which the ironis substantially free of zinc, aluminum, copper, and carbon.
 9. Theapparatus of claim 1 in which the solution is a photographic fixersolution.
 10. The apparatus of claim 1 in which the substrate is fromabout 1/8 inch to about 1/2 inch in diameter.
 11. The apparatus of claim10 in which the substrate is from about 1/4 inch to about 3/8 inch indiameter.
 12. Apparatus for recovering silver from a photographic fixersolution comprisinga container formed of a material substantiallynonreactive with the fixer solution, an inlet for introducing the fixersolution into the container, an outlet for removing spent fixer solutionwith silver removed therefrom from the container, and a metallicexchange medium within the container, the metallic exchange mediumcomprising expanded polystyrene beads coated with sponge iron of aparticle size of from about +100 mesh to about -10 mesh, the volume ofthe metallic exchange medium comprising from about 70 to about 90% ofthe volume of the container.
 13. The apparatus of claim 12 in which thesponge iron is of a particle size of from about +80 mesh to about -20mesh.
 14. The apparatus of claim 1 or 12 wherein the inlet and theoutlet are located so that solution is introduced to the bottom of thecontainer and spent solution is withdrawn from the top of the container.15. The apparatus of claim 12 in which the beads are from about 1/8 inchto about 1/2 inch in diameter.
 16. The apparatus of claim 15 in whichthe beads are from about 1/4 to about 3/8 inch in diameter.
 17. A methodfor removing silver from a silver-containing solution comprising thesteps of:(a) selecting an apparatus comprising a container formed of amaterial substantially non-reactive with the solution, the containercontaining a metallic exchange medium comprising a plurality of bouyantelements each comprising a substrate coated with iron, the substratebeing substantially non-reactive with the solution; (b) introducing thesolution into the container, wherein the silver and iron undergo anelectrochemical exchange reaction; and (c) recovering spent solution ofreduced silver content from the container.
 18. The method of claim 17 inwhich the volume of the metallic exchange medium is from about 70 toabout 90% of the volume of the container.
 19. The method of claim 17 or18 in which the solution is introduced to the bottom of the containerand removed from the top of the container.
 20. The method of claim 17 inwhich the substrate is a polymeric pyrolyzable material.
 21. The methodof claim 20 in which the iron is sponge iron.
 22. The method of claim 21in which the sponge iron is of a particle size of from about +150 meshto about -10 mesh.
 23. The method of claim 22 in which the sponge ironis of a particle size of from about +80 mesh to about -20 mesh.
 24. Themethod of claim 17, 20, 21, 22 or 23 including the additional steps ofsubsequently removing the contents of the container from the container,drying the removed contents, and pyrolyzing the substrate to leavesubstantially only iron and silver particles that can be refined withoutcomminution.
 25. The method of claim 24 in which the steps of drying andpyrolyzing comprise heating the removed contents to a sufficiently hightemperature for a sufficient time to both dry and pyrolyze the contentssimultaneously.
 26. A method for recovering silver from a solutioncomprising the step of contacting the solution with a metallic exchangemedium comprising a plurality of buoyant elements comprising a substratesubstantially non-reactive with the solution and an iron coating, theelements being sufficiently buoyant to float in the solution.